The present invention relates to variable flux transformers and, more particularly, to such a transformer which enables the magnetic coupling between the primary and secondary electrical coils of the transformer to be finely adjusted between a maximum coupling and an essentially zero coupling and yet is simple in design, manufacture and operation.
Variable flux transformers have long been available for use in situtations in which it is desired that the electromotive force (emf) output of a transformer be variable relative to its emf input. Typically such a transformer is designed to allow adjustment of the magnetic coupling between its primary and secondary electrical coils, i.e., adjustment of the amount of the magnetic flux generated by the primary coil which can interact with a secondary coil to generate an output emf. Since the value of such output emf will be directly dependent upon the flux flow change responsible for the same, the result is that the electrical output of the secondary can be correspondingly varied.
For various reasons, presently available variable flux transformers are not suited for many uses in which their function is desirable. For example, most transformers of this type now available are relatively expensive due to intricacies involved in manufacturing the same. The magnetic core which ties together the primary and secondary coils is often manufactured in separate pieces on which the coils are individually spun wound and which then must be assembled to achieve good magnetic flow therebetween. That is, the core of many variable flux transformers has a configuration which prevents the coils from being installed thereon without a bobbin after the core is constructed. This is a relatively expensive coil winding technique. Most transformer manufacturers therefore believe it necessary to manufacture a variable flux transformer core in separate pieces which have coils wound thereon and then must be joined with a joint that does not hinder good magnetic flux flow. Another problem with many variable flux transformers is that they also do not provide sufficient or efficient variation in flux coupling for potential uses requiring low values of secondary emf. In this connection, most variable flux transformers use a bypass path or magnetic route to accommodate flux generated by the primary coil which is not to be passed by the secondary coil. In most of such designs the secondary remains at all times magnetically coupled to the primary through, for example, a magnetic circuit which is parallel to the bypass circuit. The result is that when low secondary emf is desired and the bypass path is selectively closed for good magnetic flow to accommodate the full amount of the flux generated by the primary, some flux will still flow through the secondary parallel circuit to interact with the secondary coil and induce a small unwanted electromotive force. This unwanted emf has prevented selection of low secondary emf outputs with any accuracy. While some designs allow the secondary magnetic circuit to be opened completely and thus avoid this problem, these designs invariably merely transfer both the parallel bypass circuit and the accuracy problem to the maximum secondary emf output condition of the transformer.